Process for the continuous production of thin, elongated, single crystals of zinc oxide and antimony tri-oxide



.1. E. GORDON ETAL 3,096,155 PROCESS FOR THE CONTINUOUS PRODUCTION OFTHIN, ELONGATED, SINGLE CRYSTALS OF ZINC July 2, 1963 OXIDE AND ANTIMONYTRI-OXIDE Filed Nov. 25, 1959 FIG.|.

FIG.2.

PROCESS FOR THE CONTINUOUS PRODUCTION OF THIN, ELONGATED, SINGLECRYSTALS F ZINC OXIDE AND ANTIMONY TRI-OXIDE James Edward Gordon,Christopher Charles Evans, and

Noel James Parratt, all of Safiron Walden, England, asslgnors to T. I.(Group Services) Limited, Aston, England, a company of Great BritainFiled Nov. 25, 1959, Ser. No. 855,431 Claims priority, application GreatBritain Nov. 26, 1958 1 Claim. (Cl. 23144) This invention is concernedwith the production of whiskers for use as reinforcement. It is knownthat various materials, when in the form of extremely thin elongatedsingle crystals known as whiskers," have strengths many time that of thebulk material, and approaching the theoretical maximum strength deducedfrom considerations of inter-atomic forces. Accordingly, it has beenproposed to use such whiskers as reinforcement for a matrix in whichthey are embedded.

Various methods are known for the production of whiskers, but hithertothese have only been on a laboratory scale and not suited to quantityproduction. Such methods include the subjection of a metal to extrememechanical pressure, the chemical reduction of metal compounds,deposition from vapour or firom solutions, and electrolysis.

:It is an object of the invention to enable whiskers to be produced inan economical manner on a commercial scale such as to make their use asreinforcement in commercially produced materials a practicalpossibility.

It is now proposed, according to the invention, to produce whiskers on acommercial scale by carrying out one of the above processes, such asdeposition from solution or from vapour, or chemical reduction, in acontinuous manner in a chamber from which the grown crystals aredeposited continuously by the action of gravity or are otherwise removedwhen they have reached the desired size. For example, a vapour may becirculated through a chamber similar to a vertical shaft furnace, thechamber being subjected to a light upward current of The conditions inthe chamber are arranged to be such as to promote crystal growth, whichmay be initiated by artificial nucleation at the input end. The vapouris continuously recirculated and only those crystals exceeding a certainsize will have a sufficient weight in relation to their air resistancesto overcome the upward current and be deposited. They may be harvestedcontinuously from the bottom of the chamber by means of a conveyor, onwhich they are oriented before incorporation in a matrix to form astructural material.

The chamber may be arranged horizontally, with the incoming vapour orsolution entering on one side and leaving from the opposite side, andany crystals which reach the desired minimum size fall out of thehorizontal current into the stagnant region below to be collected.

Where the whiskers are produced by a chemical reaction, it is necessaryto control the speed at which the reaction takes place, since if thereaction occurs too rapidly, a powder is formed, the whiskers havinginsufiicient time to grow. The speed is controlled by limiting thesupply of one of the reactants, for example, by diluting the reactantsin a stream of inert gas.

The invention will now be described by way of example with reference tothe accompanying drawings, in which:

FIGURE 1 shows one form of apparatus suitable for producing whiskers bya straightforward physical evaporation process; and

FIGURE 2 shows apparatus suitable for producing whiskers by a continuouschemical reaction.

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Three examples of processes according to the invention will bedescribed.

Example I This is an example of the production of whiskers of copperphthalocyanine by physical evaporation using the apparatus of FIGURE 1.Dry carbon dioxide is blown at a rate of 6 litres per minute into aheating zone A which is maintained, by thermostatic control means notshown, at a temperature of 760 C. With the carbon dioxide there is bornealong one gram per minute of commercial copper phthalocyanine powder andthis sublimes to a vapour in the zone A. The zone A leads to a chamberE, of which, in the zone B, the walls are maintained at a temperature ofC. The majority of the phthalocyanine vapour is carried by the gasstream into this zone B, and the vapour condenses out into whiskers. Thechamber E has an outlet C at its upper end, and so there is a generalupward current of gas to this outlet, and the dimensions of the chamberE are such that this upward current has a mean velocity of approximately1 cm. per second in the zone B. Thus the light whiskers are heldsuspended in this zone, but as they grow and become heavier the upwardcurrent is unable to support them and they fall down to the base D ofthe chamber E, from which they can be withdrawn from time to time. Itwill be understood that in this way only whiskers or bunches of whiskersof a certain predetermined minimum size reach the base D for collection.

Example II This is an example of a combined reaction and condensationprocess for producing whiskers of antimony trioxide, using apparatuswhich is simple enough not to need illustration. A stream of air ispassed over a bath of molten antimony at a temperature of 800 C. and theoxygen in the air reacts with the antimony to form antimony trioxidevapour, with which the air stream becomes saturated. This saturatedstream is then passed into a large condensing chamber held at 550 C. andthe antimony trioxide condenses out in the form of regularly shapedneedles or whiskers. These needles or whiskers are carried out of thecondensing chamber by the velocity of the gas stream and the stream isthen passed to a filter in which they are collected, or into a furtherchamber in which the gas velocity is reduced to a point at which theWhiskers fall out under gravity and are collected.

Example III This is an example of the production of whiskers bycontinuous chemical reaction, using the apparatus of FIG- URE 2.Metallic zinc -F is boiled in a side chamber G and is carried, by meansof a gentle flow or argon gas passing through that chamber, into a mainreaction chamber H in which the atmosphere is largely of argon. Thechamber H is maintained at a temperature of 1000 C. Around its walls arearranged nozzles K through which pure oxygen is fed in stoichiometricquantities in relation to the zinc, that is to say, in the rightproportion to the quantity of zinc entering the chamber to convert allthe zinc to zinc oxide without excess oxygen being left. The rate ofdelivery of zinc and oxygen is kept down to a level such that the zincoxide has time to grow in whiskers and is not merely formed at a largenumber of nuclei simultaneously, which would result in the formation ofzinc oxide powder. The whiskers in fact grow out in buches from commonnuclei, and these bunches or groups fall to the bottom of the chamber H,from which the whiskers can be withdrawn from time to time in .a tangledmass.

It will be appreciated that all the processes described in'the examplesare operated continuously, and this way, in spite vof the comparativelyslow growth rate of individual whiskers, it becomes commericallypractical to produce Whiskers in sufiicient quantities for use asreinforcing fibers in otherimaterials and -to'=achieve thereby a finalproduct at a competitive price. The invention' is not limited to anyparticular materials for the Whiskers, as the process may be carried outwith any materials that undergo whisker formation under the rightconditions, and the parameters such astte'mperatur'aipressure, and rateof delivery of the reactants mustlobviously he selected to suit' thereactants involved. "These parameters also exercise some control overthe size and length/diameter ratio of the resulting whiskers. l

' We claim: I

A process for the continuous production of thin elongated, singlecrystals of the classknown as whiskers comprising the steps ofsubjecting a metallic compound selected from the group consisting ofoxide, and antimony tri-oxide to heat in a heating zone which ismaintained at'a temperature above'the vaporizing temperature of saidcompound, continuouslyv passing vaporized compound formed in saidheating zone from said zone into a region of a condensing vessel,maintaining said region at a temperature below the vaporizingtemperature of said compound, continuously passing an upward current ofinert gas through said region, controlling the mean velocity of saidupward current so that it initially maintains in suspension condensedcrystals of said compound formed in said gas in said region as a resultof the lowered temperature thereof until said crystals have grown to asize wherein they fall by gravity against said upward current of gas,and collecting from the bottom of the vessel the crystals of metalliccompound that are formed and fall under gravity. 7

References Cited in the file of this patent UNITED STATES PATENTS Trumpet a1 May 12,1912 Kaufiman et a1. June 1-8, |1957 OTHER REFERENCESBrenner articles in Acta Metallurgica, vol. 4, January ,1956, pages62-64, 74, and vol. 4, May 1956, pages 268- 270. 1

